The nitrenium ion is generally accepted to be the ultimate carcinogen derived from aromatic amines. Few studies have been made on these species in simple well defined chemical systems. The object of this research is to make a careful study of the individual rate constants involved in the reactions of these species (or their precursors) with simple nucleophiles. Data will be obtained for derivatives of the complex carcinogenic aromatic amines as well as for simple para substituted anilines. Analysis of the data will be made in order to: (a) assess those factors responsible for substrate and positional selectivity in the nitrenium ion, and (b), determine whether relative rate constants for specific nucleophiles correlate with carcinogenic activity. This latter technique appears to provide a useful index of carcinogenicity for the polycyclic arene oxides.